fix sync: INSERT OR IGNORE for duplicate messages, fix leaked IMAP continuations

- insertMessages uses INSERT OR IGNORE on (mailboxId, uid) conflict instead of
  crashing on UNIQUE constraint when messages are re-fetched after restart
- IMAPResponseHandler.sendCommand resumes any leaked previous continuation before
  registering a new one, preventing DuplicateCommandTag errors
- add channelInactive handler to resume pending continuations on connection drop
- add error type to sync failure log for better diagnostics

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Packages/MagnumOpusCore/Sources/IMAPClient/IMAPResponseHandler.swift

@@ -53,6 +53,15 @@ final class IMAPResponseHandler: ChannelInboundHandler, RemovableChannelHandler,
 		}
 	}
 
+	func channelInactive(context: ChannelHandlerContext) {
+		let error = IMAPError.notConnected
+		continuation?.resume(throwing: error)
+		continuation = nil
+		greetingContinuation?.resume(throwing: error)
+		greetingContinuation = nil
+		context.fireChannelInactive()
+	}
+
 	func errorCaught(context: ChannelHandlerContext, error: Error) {
 		continuation?.resume(throwing: error)
 		continuation = nil
@@ -68,6 +77,11 @@ final class IMAPResponseHandler: ChannelInboundHandler, RemovableChannelHandler,
 	}
 
 	func sendCommand(tag: String, continuation cont: CheckedContinuation<[Response], Error>) {
+		// Resume any leaked continuation from a previous command to avoid
+		// "SWIFT TASK CONTINUATION MISUSE: leaked its continuation"
+		if let old = continuation {
+			old.resume(throwing: IMAPError.serverError("Previous command interrupted"))
+		}
 		expectedTag = tag
 		continuation = cont
 		buffer = []